1. Field of the Invention
This invention relates to the production of hydrocarbon oils from coal and, more particularly to a novel process involving the low-temperature depolymerization and liquefaction of coal whereby the depolymerization is achieved through a sequence of processing steps.
2. The Prior Art
Coals vary in rank from peats to anthracites with a spectrum of grades in between such as lignites, sub-bituminous and bituminous coals. The fossilized remains of plant structures in coal indicate that plants were the source material for the coal. It has been commonly assumed that coals were formed by a variety of biodegradative and geochemical transformations of plant debris that have taken place over an extended period of time. The rank of the coal depends on the length and rate of the coalification process. The progress of the coalification of coal from lignite to anthracite results in a general decrease in hydrogen and oxygen contents of the organic matter. Carbon content, on the other hand, increases from about 70% and below in lignite to over 90% in anthracite. Oxygen functionality also varies with rank.
Because of its complexity, it is nearly impossible to assign a specific molecular structure to coal. There is no uniform repeating monomer unit in coal such as is found in saccharides, proteins, and cellulose. Results and interpretations derived from numerous studies of coal liquids, produced by high temperature liquefaction processes, have led to tentative proposals on the structure of coal. A general concensus has been reached that coal is made up of a variety of condensed naphthenoaromatic ring systems designated as "clusters" which are interconnected by linking groups, e.g., etheric groups and short (C.sub.1 -C.sub.3) alkylene chains. It has also been indicated that coal contains short aliphatic side chains and heteroatoms.
During the 1960's and 1970's sustained efforts to improve and upscale some of the more promising liquefaction procedures were made. Examination of available publications and reports indicates, however, that in most cases optimization was sought mainly by improvement of the engineering aspects of these processes, with relatively lesser attention paid to the possibility of major modifications based on better understanding and control of the critically important organic-chemical aspects of coal liquefaction. This approach apparently did stem to a large extent from the insufficient knowledge on coal structure at a molecular level, as well as from a widely accepted belief that coal can be transformed into a desirable range of liquid products by application of drastic operating conditions, irrespective of its exact chemical structure and inherent chemical properties. The scientific inadequacy of this approach is best illustrated by the marked lack of novelty and imagination in catalyst development for coal liquefaction during the above indicated period.
Both physical and chemical methods have been extensively used in the investigation of coal structure. Physical studies have included application of spectral methods, e.g., X-ray scattering, ultraviolet and visible spectroscopy, reflectance, C-13 nuclear magnetic resonance (CMR), etc., as well as determination of physical properties, e.g. molar refraction, electrical conductivity, molar diamagnetic susceptibility, molar volume, dielectric constant, sound velocity, thermal stability, etc.
Parallel to the work on the engineering improvement of coal liquefaction processes, a large number of studies concerned with the organic chemistry of coal have been reported in the literature. These studies have significantly contributed to the understanding of the chemical functionality of coal, and have provided information on certain types of organic reactions which could be used to affect the extent of its solubilization. With few exceptions, a more or less similar coal-structural working model was used by the above authors in interpretation of results obtained. The model suggested consists of rather small (2- to 5-ring) naphthenoaromatic or naphthenoaromatic-heterocyclic condensed systems (clusters) interconnected by different types of linking groups. The size of the clusters, i.e., the number of condensed rings per cluster, increases with increase in coal rank. The proportion of aromatic and hydroaromatic rings in the clusters also depends on the rank of the coal. Catalytic dehydrogenation and other methods have been previously used to derive tentative estimates of alicyclic ring contents in coal. These estimates have been generally low, e.g., up to 20%, as compared to recent and more reliable CMR data, which indicate a high proportion (40-50%) of saturated carbons in coals, primarily in naphthenic rings, and to a lesser extent in the form of alkyl and alkylene groups.
It should be noted that the proposed interlinked cluster models for coal have been usually two-dimensional. Unfortunately, consideration of three-dimensional models, and realization of the importance of steric hindrance effects in the approach of reactants or catalysts to the linking units of the coal structure, has been negligible. Close examination of the prior studies indicates that some suggested coal conversion reactions, e.g., reduction, reductive alkylation, catalytic hydrogenation or dehydrogenation, etc., affect mainly the naphthenoaromatic-heterocyclic clusters, and to a lesser extent the interlinking units. Consequently, although such reactions may lead to extensive chemical changes in the coal and attendant partial depolymerization and solubilization, the observed depth of coal breakdown into low molecular weight, monocluster components is not significant, as evidenced by the characteristically high molecular weight of the coal liquids formed. Studies concerned with the possibility of obtaining coal-structural data by selective or at least preferential cleavage of the interlinking units, for instance by reverse Friedel-Crafts reactions catalyzed by Lewis acids, have recently received increased attention.
A major part of the previously reported coal structural studies have been based on separation and identification of products obtained by coal liquefaction. It should be noted in connection with this that under the drastic operating conditions of conventional liquefaction procedures (temperature, 350.degree.-465.degree. C.; high hydrogen pressure; sulfided catalysts) there is not only an initial non-selective breakdown of the coal framework into simpler structural components but also extensive secondary chemical reactions of such primary products, resulting to an important extent in transformation of functional groups and skeletal rearrangements. Therefore, there seems to be limited value to coal structural assignments based on the composition of liquid products obtained under drastic experimental conditions.
Similar limitations in structural assignments and in coal solubilization apply to extractive liquefaction studies at moderate temperatures (275.degree.-300.degree. C.) involving the use of reactive "specific" solvents, in particular phenol and naphthols. The high reactivity of phenols at such temperatures in a variety of catalytic processes e.g., O- and C-alkylation, dienone-arenol rearrangements, Meerwein-Ponndorf reductions, etc., has been previously demonstrated. In effect, such compounds cannot be considered as solvents, in the usual sense, since they interact with coal to form products which are not related in a simple manner to the original coal structure. In other words, it is doubtful that in such cases it is possible to differentiate between products of simple coal degradation and products formed by various interactions of the phenol "solvent" with reactive components of the coal structure. Catalytic studies on coal depolymerization using phenols (at reflux temperature) as solvents are therefore also of limited value in regard to coal structural determination, or coal liquefaction.
Conventional high-temperature (&gt;375.degree. C.) coal liquefaction processes are characterized by low selectivity for light liquid products and preferential production of heavy oils, which require extensive upgrading for use as conventional fuels. Some of the basic problems associated with such processes can be attributed to the relatively limited availability and reliance on data pertaining to coal structure at a molecular level, and to the somewhat unreasonable expectation that the different types of intercluster linkages in the polymeric network of coal can be exhaustively cleaved by a single type of reaction, i.e., non-selective hydrogenolysis. Reviews covering the large volume of high-temperature (&gt;375.degree. C.) coal liquefaction studies have been recently provided (Gorin, E., in "Chemistry of Coal Utilization", 2nd Supplementary Vol., M. A. Elliot, ed., J. Wiley & Sons, New York, 1981, Chapter 27, pp. 1845-1918, and references therein).
In-depth structural analysis of products obtained by single-step metal halide-catalyzed hydrotreatment at 315.degree.-375.degree. C. of several coals, e.g., a Fruitland, N.M., coal, and a Utah Hiawatha coal, shows that even the simplest product components have a bi-cluster, i.e., incompletely depolymerized, structure. This demonstrates the limit in the depth of coal depolymerization which can be achieved by a single type of reaction, e.g., hydrotreatment.
In view of the numerous efforts to obtain a desirable coal-derived liquid from coal by means of a high temperature, single stage reaction process, and in view of the less than desirable results obtained thereby, it would be a significant advancement in the art to provide a novel, low-temperature process for the depolymerization and liquefaction of coal particularly through several sequential steps which will selectively cleave different types of bonds within the coal in each processing step. Such a novel process is disclosed and claimed herein.